Physical and biological processes in a Macrocystis pyrifera community near Valdivia, Chile

Abstract

From October 10, 1979 to August 26, 1981 we investigated the effect of physical disturbance, herbivory and algal-algal competition on the dynamics of the Macrocystis community near Valdivia, Chile. The most abundant herbivore, Tegula atra, was continuously removed from one of two sites with high initial Tegula density (control and Tegula-removal rocks) and added to one of two sites where Tegula was initially absent (Tegula-addition and addition-control rocks). Macrocystis was rare at the beginning of the study. From October 1980 to April 1981 we continuously removed Macrocystis from an additional rock near the addition-control rock. On each site the response of the algal community was monitored by photographing 9, 0.25 m² permanent quadrats at 2–4 week intervals. Tegula had little effect on Macrocystis and there were no other important herbivores present. Each summer Macrocystis dominated all 4 sites of the Tegula experiment and Tegula migrated towards the intertidal to feed. Macrocystis was reduced each winter by winter storms, the holdfasts (but not the canopy) surviving the first winter but not the second. The Macrocystis removal rock was dominated by lower growing perennial algae, indicating a competitive hierarchy among the algae of the form: Macrocystis>perennial algae>ephemeral algae>encrusting algae. Except for the presence of a competitive hierarchy among the algae and the importance of winter storms, Chilean kelp communities share few of the organizational features of their northern hemisphere counterparts. Marine mammals (excluding man) do not prey on algal consumers (Tegula and the urchin, Loxechinus albus) and these consumers have little effect on perennial macroscopic algae. Urchins apparently feed only on drift algae. Species richness in the algal community is low.